1. Field of the Invention
The present invention relates to a structure composed of alumina fibers and a process for its production. More particularly, the present invention relates to an alumina fiber structure having high mechanical strength suitable for use as a refractory heat insulating material, and a process for its production.
2. Discussion of the Background
Fibers made of an inorganic oxide such as alumina or silica have excellent heat resistance, and various structures prepared from such inorganic oxide fibers, are used as light weight refractory heat insulating materials. Such structures include products wherein fibers are bound by a binder, and non-woven products wherein fibers are laid without using any binder and bound by needling. In the non-woven products, the mechanical strength of the products derives from the intertwining of the fibers caused by the needling. However, among the inorganic oxide fibers, high alumina fibers containing at least 65% by weight of alumina are not used for a non-woven product, because it is difficult to obtain a non-woven product having high mechanical strength even when needling is applied to a laid mat of such high alumina fibers. The reason for the difficulty is not clearly understood, but the difficulty may be attributable to the fact that no adequate intertwining of such fibers takes place even when needling is applied to the mat. High alumina fibers containing at least 65% by weight of alumina are most commonly produced by a precursory fiber method. In this method, a viscous solution containing an aluminum compound as an essential component and a small amount of an organic polymer compound such as polyvinyl alcohol, is spun to form precursory fibers, and then the precursory fibers are burned at a high temperature and converted to alumina fibers, whereby high alumina fibers are produced.
Endo, one of the inventors of the present application, has found it possible to obtain a non-woven shaped product of high alumina fibers by laying, into a layered mat, precursory fibers, which would, after burning, comprise from 72 to 99% by weight of alumina and from 1 to 28% by weight of silica, needling the layered mat to obtain a non-woven structure composed of precursory fibers, and then burning the structure to convert the precursory fibers to high alumina fibers (Japanese Unexamined Patent Publication No. 88162/1985). Subsequently, Endo and Ando, another coinventor of the present invention, have found it possible to obtain a non-woven shaped product of high alumina fibers by a similar method also from precursory fibers which can be converted to high alumina fibers comprising from 65 to 72% by weight of alumina and from 28 to 35% by weight of silica, i.e. having a silica content higher than mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) (Japanese Unexamined Patent Publication No. 173151/1985).
These processes are highly evaluated in that they were the first to provide a non-woven shaped structure composed of high alumina fibers without using any binder. However, the shaped structures produced by these processes have a drawback that the tensile strength in the direction of the thickness, i.e. the peeling strength, is poor. This drawback is pronounced as the thickness of the structure increases. With a thick structure, separation is likely to occur readily at the center in the direction of the thickness even with a minimum force. According to the study made by the present inventors, the poor peeling strength of these structures is believed to be attributable to the fact that during the needling of the laid mat of the precursory fibers, the precursory fibers are likely to break when hooked and pulled by needles in the thickness direction. In other words, the precursory fibers have poor tensile strength, and when hooked and pulled by needles in the thickness direction, they tend to break one after another. Consequently, the longer the distance from the surface becomes, the smaller the number of fibers oriented by needling becomes. The peeling strength of the structure depends on the number of fibers oriented in the thickness direction by needling. Accordingly, the thicker the structure, i.e. the longer the distance from the surface of the structure to the center in the thickness direction becomes, the lower the peeling strength of the structure tends to be.